1. Field of the Invention
The present invention generally relates to a communications medium shared among several users. More particularly, the present invention relates to a method and system for detecting collisions in a shared communications medium.
2. Background Art
Communications systems that employ a shared communications medium may, in certain circumstances, permit two or more users to transmit information at the same time. For example, many time division multiple access (TDMA) communications systems designate one or more “contention time slots” within a TDMA frame structure. These contention time slots are designated for information transmissions from users according to contention-based communications protocols that allow users to transmit “at will.”
These “at will” transmission protocols can cause multiple transmissions to overlap in time upon arrival at a designated receiver (for example, at the headend of a Data Over Cable Based Service Interface Specifications (DOCSIS) network). At the receiver, such overlapping transmissions combine to form a composite signal that is termed a collision. When a collision occurs, the multiple transmissions interfere with each other in a manner that can prevent the reception of a portion or all of the information in these transmissions.
Recovery from a collision requires the retransmission of information by users. Unfortunately, as collision rates increase, so does the retransmission rate. If the number of retransmissions becomes excessive, latencies associated with the transfer of information increase and communications capacity is wasted.
To reduce the amount of wasted capacity, many communications systems are able to adjust their parameters to optimize performance. For example, certain TDMA systems are able to dynamically adjust the number of allocated contention time slots to keep collision rates, often measured in collisions per second, within an acceptable range.
Thus, to effectively control collision rates, a communications system needs to accurately detect collisions. Conventional collision detection techniques do not provide great accuracy. For example, one such technique detects collisions based solely on received signal power. According to this technique, a collision is detected when one or more power measurements are above a certain level.
This power-based technique disadvantageously assumes that all colliding signals add in power and, as a consequence, result in a power level that is greater than a power level associated with a non-colliding signal. Unfortunately, phase and timing offsets between two or more colliding signals can result in a combined signal that does not have an increased power level. In fact, such offsets may also yield combined signals exhibiting decreased power levels. Furthermore, normal transmit power variations may corrupt such collision detection processes.
Thus, power-based collision detection approaches fail to detect a substantial number of collisions. Such failures result in collision rate overestimation or underestimation, which leads to ineffective contention slot allocation decisions.
Accordingly, what is needed is a technique to determine with relative certainty when a collision has occurred in the presence of amplitude, phase, and timing offsets.